The present invention refers to an electric component, and specially to an electric component inside a multilayer printed circuit board and a method for manufacturing the same.
The invention also refers to a multilayer printed circuit board (PCB) including the electric component.
Printed circuit boards having several layers, so called xe2x80x9cmultilayerxe2x80x9d PCBs, are frequently used within the electronics. The need for an ever increasing extended number of components on smaller surfaces has resulted in a dramatic increase of the number of the layers as well as the conductors in a multilayer PCB. As is well known, a typical multilayer PCB consists of a number of insulating layers, on or between which conductors are applied. The conductive layers can among others be used as the ground and supply planes. Electric components, such as Integrated Circuits, ICs, are mounted and soldered onto one or both sides of the PCBs, for example in vias and connected to each other or to the ground or supply plane though via holes and the said conductors.
The PCB is normally provided with coupling capacitors, for example to filter interferences that may occur through rapid variations between the ground and supply plane as the result of the operation of ICs. The coupling capacitors are connected between the ground and the supply to the circuits,
Usually, the decoupling capacitors are buried in the multilayer PCB between the different layers by providing different capacitive layers between the supporting layers of the board. The board can also be provided with a capacitor layer with different capacitances when assembling circuits or other components.
One problem is to produce capacitive layers with controlled, thin layers.
It is known to use hard capacitor laminates which are placed in the multilayer constructions to create capacitive planes in which the capacitor is arranged. Also, laminates with very well defined composition and thickness are used, for instance an epoxy/ceramic laminate with 95% ceramics.
U.S. Pat. No. 5,065,284 describes production of capacitive islands in a PCB for producing decoupling capacitors. A flexible film including cells with high dielectric is used. Each capacitor consists of one or more high capacitive flexible dielectric plates provided with plated surfaces. Each dielectric plate consists of a single layer of multi-layers or a single-layer, separated from each other shivers or pearls with high dielectric constant provided between the laminae. The space between the pearls is filled with an elastic polymer or a binding agent.
According to U.S. Pat. No. 5,625,528 a monolithic capacitor circuit including several capacitors comprising several layers is produced. The capacitors are isolated from each other through double dielectric constants, three layer laminate and insulation layers. The different layers have different thermal expansion coefficients to give the capacitor circuit a better stability and cracking resistivity.
EP 545 812 discloses a plastic film to be used in capacitors. The film includes a dielectric layer, which comprises two layers, one of which has a melting temperature different from the melting temperature of the other one. The layers are extruded on each other, This document does not concern the elasticity of the materials used. Neither, the PCBs nor buried components are involved.
The main object with the present invention is to provide thin layers, preferably dielectric layers making multilayer PCB in a controlled way, in particular when producing buried capacitors or the like.
Another object of the invention is to shape the dielectric layer, in a multilayer PCBs, between chosen capacitive planes, preferably only where the dielectric is needed.
Yet another object of the invention is to create a new dielectric for capacitors on multilayer PCBs.
These tasks have been solved by the initially mentioned component including at least two conductive layers and an intermediate layer. The intermediate layer further consists of at least two layers: at least a first layer and a second layer, which at least first layer has more elastic characteristic than the second layer at a certain temperature and/or pressure.
The intermediate layer is a dielectric layer when producing capacitors. Preferably, the dielectric layer consists of two essentially separate layers: a first layer with lower melting temperature than the other layer.
When applying at least one of the conducting layers on the dielectric layer, the layer having lower melting temperature moves away and fills empty spaces and unevenness sections, while the other layer maintains a predefined thickness with desired thinness.
The first layer becomes very thin when it is forced and can adhere the second layer to at least one of the conductive layers, In one embodiment also the second layer has adhesive characteristics.
In one embodiment the intermediate layer consists of three separate layers, whereby one layer between two other layers has higher melting temperature than the two other layers. Preferably, the layers with lower melting temperature are of same material and the layer with higher melting temperature holds a controlled minimum thickness. The layers with lower melting temperature are primarily used to fill out unevenness parts and also to adhere different layers.
In one embodiment all the intermediate layers are produced as a homogenous film.
The intermediate layer contains, in one embodiment, very small amount or no amount of material with high dielectric constants Preferably, the layers in that intermediate layer consist of PTFE (polytetraflourethylene), PFA (perflouroalkoxy) and/or FEP (flourinated ethylene-propylene).
The electric component, in one embodiment, is a buried capacitor in it multilayer PCB. It may also be a resistor.
Moreover, the invention refers to a multi layer PCB, which includes a first and a second laminate and a number of layers constituting a buried electric component. The buried component includes a first and a second conductive layer and one intermediate layer consisting of at least two layers. At least one layer at a certain temperature and/or pressure is more elastic than the other layer.